The present invention relates to powered respirators, by which is meant respirators of the kind where a pump provides a forced flow of filtered air to the face of the wearer to ensure (in normal operation) that he has an adequate supply of clean breathable air when worn in a dusty or otherwise contaminated environment--for example as described in United Kingdom patent specification no. 1495020 and of which examples are marketed by the assignee of the present application under the trade marks AIRSTREAM and JUPITER. Such respirators are generally arranged to deliver air at a sufficient rate to maintain a slight positive pressure in the region of the user's face throughout the breathing cycle, so that there can be no inward leakage of contaminated air from the surrounding environment. However, circumstances can sometimes arise in which this pressure will not be maintained at a superatmospheric level during inhalation--with a consequent danger of contaminants being breathed in--for example if the user is breathing at an exceptionally high rate or if the airflow is reduced due to filter blockage or battery exhaustion. In such circumstances it is desirable to provide some means of warning the user that the supply pressure is approaching a negative condition in order that he can take appropriate corrective action.
In the past attempts to devise such warning devices for powered respirators have generally been based on monitoring the rate of flow of air delivered by the pump, rather than by monitoring the actual pressure within the user's visor or mask. However, flow measurement is difficult and expensive and in any event may not give a true indication that the pressure as supplied to the user has fallen below a safe level, e.g. if the user is breathing particularly hard or if air from the pump is being dumped to the atmosphere through an ill-fitting or raised visor. It is recognised, therefore, that there is a need for a device which is capable of monitoring the pressure within the visor or mask of a powered respirator and of responding to even slight departures of that pressure below a selected level.
In United Kingdom patent specification no. 2130893 there is disclosed a performance monitor for a powered respirator helmet in the form of a float-type pressure gauge mounted inside the visor of the helmet and comprising a ball which changes its position along the length of a transparent tube in dependence upon the pressure within the visor. Such a gauge is, however, extremely difficult for the user to monitor effectively at anything other than odd intervals as he must adjust his field and focus of view specifically to the gauge in order to accurately assess the position of the ball at any time; indeed, the presence of this gauge may act as an unwelcome distraction to the user. The normal head movements of the user may also disturb the position of the ball. It is furthermore impossible for this kind of monitor to give an audible warning to the user in the event of a pressure reduction below a selected level, which form of warning is much to be preferred as the most practical way of drawing his attention to this potentially hazardous situation.